Motor data

Speeds, efficiency, fuses, code letters, insulation and motor types.

Motor speed by poles

Poles	60 Hz synchronous	60 Hz full load	50 Hz synchronous	50 Hz full load
2	3600	about 3450	3000	about 2850
4	1800	about 1750	1500	about 1450
6	1200	about 1150	1000	about 950

RPM = 120 × Hz ÷ poles. Slip takes about 4 to 5% off at full load. A motor wound for one frequency should run only on that frequency.

Premium-efficiency minimums

Motor size (hp)	2 pole	4 pole	6 pole
1	77.0%	85.5%	82.5%
1.5	84.0%	86.5%	86.5%
2	85.5%	86.5%	87.5%
3	85.5%	89.5%	88.5%
5	86.5%	89.5%	89.5%

Large motors reach about 97% efficient. Small motors are often only 50 to 60%.

Largest fuse for small motors

Motor size (hp)	120 V	240 V
1/6	4.4 A	2.2 A
1/4	5.8 A	2.9 A
1/3	7.2 A	3.6 A
1/2	9.8 A	4.9 A
3/4	13.8 A	6.9 A
1	16.0 A	8.0 A
1 1/2	20.0 A	10.0 A

Fast-acting fuses suit blowers. Compressors and other high-starting-current motors need time-delay (dual-element) fuses, which ride through about 10 seconds of overload.

Insulation class and winding temperature

Class	Highest winding temperature
A	221 °F (105 °C)
B	266 °F (130 °C)
F	311 °F (155 °C)
H	356 °F (180 °C)

Locked-rotor code letters

Letter	kVA per hp
A	0 to 3.14
B	3.15 to 3.54
C	3.55 to 3.99
D	4.0 to 4.49
E	4.5 to 4.99
F	5.0 to 5.59
G	5.6 to 6.29
H	6.3 to 7.09
J	7.1 to 7.99
K	8.0 to 8.99
L	9.0 to 9.99
M	10.0 to 11.19
N	11.2 to 12.49
P	12.5 to 13.99
R	14.0 to 15.99
S	16.0 to 17.99
T	18.0 to 19.99
U	20.0 to 22.39
V	22.4 and up

kVA per horsepower with the rotor locked. Locked-rotor amps are usually 2 to 6 times full-load amps.

Single-phase motor types

Type	How it starts	Typical use
Split-phase	Start winding dropped out at about 75% speed	Fans and small units up to 1/3 hp; cap-tube systems whose pressures equalize
CSIR	Start capacitor in series with the start winding, dropped out at about 75% speed	Hermetic refrigeration compressors up to 3 hp
CSCR	Start capacitor (switched) plus a run capacitor that stays in	Large commercial hermetics and belt-drive blowers (1/6 hp and up)
PSC	One run capacitor, no switch. Low starting torque	Multi-speed blowers and fans; easy-start compressors with equalized pressures. Sensitive to low voltage
Shaded pole	Copper bands on the poles, no start winding	Small fans, 1/100 to 1/6 hp
Three-phase	No start winding or capacitor	1 hp and up. Dual voltage motors have 9 leads: series for 480 V, parallel for 240 V
ECM	Brushless DC motor with an electronic module	Blowers and fans with soft start and variable speed

Temperature and nameplate rules

The hottest spot in a motor should be no more than about 72 °F above ambient. A typical maximum internal temperature is about 150 °F.
Internal temperature is about the frame temperature (infrared thermometer) plus 20 °F.
Internal overload protectors open at about 200 to 250 °F. After a trip a hermetic motor can take 1 to 2 hours to reset.
A 5 to 10% voltage drop makes a PSC motor hard to start and overheated.
Replacement motor: same specs, and the current should not be lower than the original (up to 10% higher is allowed).
Start winding drops out at about 60 to 75% of full speed.
Nameplate items to check: voltage, full-load amps, hertz, phase, RPM, hp, insulation class, duty (continuous or intermittent), code letter, service factor and thermal protection.
Greasing: a small amount of grease every 2 to 3 months on greaseable ball bearings, with the drain plug out, then run 10 to 30 minutes. Wick-oiled sleeve bearings need 1 to 2 drops every 6 months.

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Calculation aids and reference figures for licensed technicians. Always follow the manufacturer’s data plate and manual for the exact model, and local codes. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Pressures are typical saturation values and change with load and airflow.